Styrenic block copolymers, such as SEBS (polystyrene-saturated polybutadiene-polystyrene), SBS (polystyrene-polybutadiene-polystyrene), SEPS (polystyrene-saturated polyisoprene-polystyrene), SIS (polystyrene-polyisoprene-polystyrene), and SEPSEP are known in the art. They exhibit excellent physical properties, such as elasticity and flexibility. However, they often cannot be readily processed on typical polyolefin processing equipment, without the need for flow enhancers and other processing aids. Upon formulation with such materials, end-use properties such as tensile strength and heat resistance can suffer. Furthermore, they can suffer from thermal instability such as cross-linking (i.e. SBS) and scission (i.e. SIS).
Ethylene/α-olefin multi-block interpolymer compositions are readily processible using typical polyolefin processing equipment. They exhibit desirable end-use properties such as high heat resistance and high tensile strength. However, ethylene/α-olefin multi-block interpolymer compositions are typically are not as flexible and elastic as the most elastic styrenic block copolymers when used at high strains (i.e. >100%).
It would be desirable to have a thermoplastic elastomer composition which exhibits excellent physical properties, such as elasticity and flexibility, while at the same time being readily processible using typical polyolefin processing equipment.
Elastomeric compositions have found particular use in elastic films and fibers. They can be used by themselves, but are more commonly used in laminated structure wherein the substrate is a nonwoven fabric. The elastic film or fiber imparts elasticity to the nonwoven laminates. Such elastic nonwoven laminate materials have found use in the hygiene and medical market particularly in such applications as elastic diaper tabs, side panels of training pants, leg gathers, feminine hygiene articles, swim pants, incontinent wear, veterinary products, protective clothing, bandages, items of health care such as surgeon's gowns, surgical drapes, sterilization wrap, wipes, and the like. These materials may also find use in other nonwoven applications including but are not limited to filters (gas and liquid), automotive and marine protective covers, home furnishing such as bedding, carpet underpaddings, wall coverings, floor coverings, window shades, scrims etc.
Elastomeric films can be made in a number of ways known to those of ordinary skill in the art. Single or multi-layer elastic films are possible. Such processes can include bubble extrusion and biaxial orientation processes, as well as tenter frame techniques. In order to facilitate elasticity, the elastic film is usually employed singly or as a layer, in the case of multi-layer films. As many elastic compositions tend to be sticky and hence difficult to process or poor in hand feel, at least one non-sticky/non-tacky material may be used to comprise at least a portion of the film surface to mitigate this effect. Alternatively, various additives and modifiers such (i.e. slip agents, anti-block etc.) may also be employed.
Elastomeric fibers or filaments can be made in a number of ways known to those of ordinary skill in the art. Monofilament, bicomponent, multicomponent, islands-in-the-sea, crescent, side-by-side and other configurations known to those of ordinary skill in the art are suitable for use with the inventive composition. Like films, elastic fibers also tend to be sticky and hence difficult to process or poor in hand feel. At least one non-sticky/non-tacky material may be used to comprise at least a portion of the film or fiber surface to mitigate this effect. Alternatively, various additives and modifiers such (i.e. slip agents, anti-block etc.) mentioned above may also be employed.
Elastomeric compositions are often used for their retractive force. They are commonly used in various forms including fibers, film, laminates and fabric. When used in an article, the retractive force of the elastomer provides the “holding force” of the structure. For example, in health and hygiene articles such as infant diapers, training pants, and adult incontinence articles, the elastomers are commonly used in laminate structures. These laminate structures help to maintain fit of the article to the body. Body heat can result in the decrease of the holding force of the elastomer over time (measured as stress-relaxation) which can translate to loosening and eventual sagging of the article resulting in a decrease in fit. Styrenic block copolymers and their formulations used in these structures can suffer from excessive stress-relaxation and consequently articles fabricated using these materials can sag unacceptably in end-use. Accordingly, it is a goal to reduce the amount of stress-relaxation (increase in heat resistance) of the elastomer. This phenomenon is a known problem and has been described previously in the art. Such art includes but is not limited to WO9829248A1, WO0058541A1, US20020052585A1, US20040127128A1, U.S. Pat. No. 6,916,750B2, U.S. Pat. No. 6,547,915B2, U.S. Pat. No. 6,323,389B1, U.S. Pat. No. 6,207,237B1, U.S. Pat. No. 6,187,425B1, U.S. Pat. No. 5,332,613A, U.S. Pat. No. 5,288,791A, U.S. Pat. No. 5,260,126A, U.S. Pat. No. 5,169,706A, GB2335427A, and WO2004037141A1.
The compositions suitable for use in elastic films and laminates comprise a polymer blend comprising at least one ethylene/α-olefin interpolymer, wherein the ethylene/α-olefin interpolymer:
(a) has a Mw/Mn (Mw denotes weight averaged molecular weight; Mn denotes number averaged molecular weight) from about 1.7 to about 3.5, at least one melting point, Tm, in degrees Celsius, and a density, d, in grams/cubic centimeter, wherein the numerical values of Tm and d correspond to the relationship:Tm>−2002.9+4538.5(d)−2422.2(d)2; or
(b) has a Mw/Mn from about 1.7 to about 3.5, and is characterized by a heat of fusion, ΔH in J/g, and a delta quantity, ΔT, in degrees Celsius defined as the temperature difference between the tallest DSC peak and the tallest CRYSTAF peak, wherein the numerical values of ΔT and ΔH have the following relationships:ΔT>−0.1299(ΔH)+62.81 for ΔH greater than zero and up to 130 J/g,ΔT≧48° C. for ΔH greater than 130 J/g,wherein the CRYSTAF peak is determined using at least 5 percent of the cumulative polymer, and if less than 5 percent of the polymer has an identifiable CRYSTAF peak, then the CRYSTAF temperature is 30° C.; or
(c) is characterized by an elastic recovery, Re, in percent at 300 percent strain and 1 cycle measured with a compression-molded film of the ethylene/α-olefin interpolymer, and has a density, d, in grams/cubic centimeter, wherein the numerical values of Re and d satisfy the following relationship when ethylene/α-olefin interpolymer is substantially free of a cross-linked phase:Re>1481-1629(d); or
(d) has a molecular fraction which elutes between 40° C. and 130° C. when fractionated using TREF, characterized in that the fraction has a molar comonomer content of at least 5 percent higher than that of a comparable random ethylene interpolymer fraction eluting between the same temperatures, wherein said comparable random ethylene interpolymer has the same comonomer(s) and has a melt index, density, and molar comonomer content (based on the whole polymer) within 10 percent of that of the ethylene/α-olefin interpolymer; or
(e) has a storage modulus at 25° C., G′(25° C.), and a storage modulus at 100° C., G′(100° C.), wherein the ratio of G′(25° C.) to G′(100° C.) is in the range of about 1:1 to about 9:1.
and, at least one styrenic block copolymer;
wherein the ethylene/α-olefin interpolymer has a density of from about 0.855 to about 0.878 g/cc and a melt index (I2) of from about 0.5 g/10 min. to about 20 g/10 min.